A transparent conductive film is used in various fields such as a liquid crystal display (LCD), a plasma display panel (PDP), an organic electroluminescence (OLED), a transparent electrode for a photovoltaic cell (PV) and a touch panel (TP), an electro-static discharge (ESD) film, and an electromagnetic interference (EMI) film, etc. For the transparent conductive film, (1) low surface resistance, (2) high light transmittance, and (3) high reliability, are required.
For example, a transparent electrode for an LCD has a surface resistance within a range of 10 to 300Ω/□, and a light transmittance in the visible light range of 85% or more. More preferably, the surface resistance is within the range of 20 to 100Ω/□, and the light transmittance is 90% or more. A transparent electrode for an OLED has a surface resistance within a range of 10 to 100Ω/□, and alight transmittance in the visible light range of 80% or more. More preferably, the surface resistance is within the range of 10 to 50Ω/□, and the light transmittance is 85% or more. A transparent electrode for a PV has a surface resistance of within a range of 5 to 100Ω/□, and a light transmittance in the visible light range of 65% or more. More preferably, the surface resistance is within the range of 5 to 20Ω/□, and the light transmittance of 70% or more. An electrode for a TP has a surface resistance within a range of 100 to 1000Ω/□, and alight transmittance in the visible light range of 85% or more. More preferably, the surface resistance is within the range of 150 to 500Ω/□, and the light transmittance in the visible light range is 90% or more. An ESD film has a surface resistance within a range of 500 to 10000Ω/□, and a light transmittance in the visible light range of 90% or more. More preferably, the surface resistance is within the range of 1000 to 5000Ω/□, and the light transmittance in the visible light range is 95% or more.
Conventionally, ITO (Indium Tin Oxide) has been used for transparent conductive films used for such transparent electrodes. However, indium used for ITO is a rare metal, and recently, stabilizing the supply and the price of indium has become an issue. Also, for the formation of ITO films, a sputtering method, a vapor-deposition method, and the like requiring high vacuum are used, and thus, a vacuum production apparatus is required and the production takes a long time, resulting in the higher cost. Further, a crack may be easily generated in ITO due to a physical stress such as bending and ITO can be easily broken. Therefore, applying ITO to a flexible substrate is difficult. Accordingly, alternative materials for ITO capable of overcoming these drawbacks have been searched for.
Among “alternative materials for ITO”, as materials which do not require the use of a vacuum production apparatus, and which can be used for forming films by coating, conductive materials, for example, (i) polymer-based conductive materials such as poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonic acid) (PEDOT: PSS) (for example, refer to Patent Document 1), conductive materials containing a nanostructured conductive component such as (ii) metal nanowires (for example, refer to Patent Document 2 and Non-patent Document 1), and (iii) carbon nanotubes (for example, refer to Patent Document 3), have been reported.
Among these, (ii) conductive materials containing metal nanowires are reported to have a low surface resistance and a high light transmittance (for example, refer to Patent Document 2 and Non-patent Document 1), and further, have flexibility, and thus, are suitable as “alternative materials for ITO”.
Here, pattern formation of a transparent conductive film is required corresponding to the purpose of use, in order that the transparent conductive film is used as a transparent electrode. For the pattern formation of ITO, a photolithography using a resist material has been generally used.
As a method for forming a conductive material containing metal nanowires, Non-patent Document 1 describes the following steps.    (1) Step for coating a conductive ink containing metal nanowires on a substrate    (2) Step for forming a transparent conductive layer by sintering    (3) Step for forming a photosensitive resist on the transparent conductive layer    (4) Step for applying photoenergy to the resist through an appropriate light shielding mask corresponding to a fine pattern    (5) Step for developing the obtained latent image of the resist with an appropriate development solution    (6) Step for removing the exposed patterned film (transparent conductive layer) by an appropriate etching method    (7) Step for removing a remaining resist using an appropriate method
In practice, in addition to the above steps, appropriate substrate surface treatment, washing, and drying steps, etc., may be required.
Patent Document 2 describes the following steps.    (1) Step for coating a conductive ink containing silver nanowires dispersed in water, on a substrate    (2) Step for forming a silver nanowire network layer by sintering    (3) Step for depositing a photo-curable matrix material containing a prepolymer on the silver nanowire network layer    (4) Step for applying photoenergy to the matrix material through a light shielding mask corresponding to a fine pattern    (5) Step for removing uncured regions by washing with a solvent (ethanol), or step for physically removing the uncured regions using an adhesive tape or a tacky roller.
Patent Document 4 describes the following steps.    (1) Step for coating a conductive ink containing silver nanowires dispersed in a solution containing a photopolymer, on a substrate    (2) Step for applying photoenergy to the photopolymer through an appropriate light shielding mask corresponding to a fine pattern    (3) Step for developing uncured regions with a developing solution.
Patent Document 5 describes the following steps.    (1) Step for coating a resin solution containing metal nanofillers on a surface of a transparent base material    (2) Step for arranging a mask having a patterned opening, on the surface of the transparent conductive film    (3) Step for performing a plasma treatment or a corona treatment through the mask on the side opposite to the side of the transparent conductive film, for forced oxidization of the metal nanofillers in the transparent conductive film at the portion corresponding to the opening of the mask, and forming a non-conductive portion with the oxidized metal nanofillers and forming a conductive portion with the unoxidized metal nanofillers.
However, in either of the methods disclosed in Non-patent Document 1 and Patent Document 2, a step for further forming a photosensitive layer for pattern formation, on the layer containing the metal nanowires, is necessary. Further, a step for developing the photosensitive layer, and a step for removing the exposed metal-nanowire containing layer, are necessary. Thus, the silver nanowires at the portion to be removed are wasted, waste disposal of the developer may be required. In addition, after the development of the photosensitive layer and the removal of the exposed metal-nanowire containing layer, a step for removing the photosensitive layer may be required.
The method disclosed in Patent Document 4 also requires a development step, and thus, the silver nanowires may be wasted by the development, and a problem regarding the waste disposal of the developer may occur.
Accordingly, in order to obtain a transparent conductive film, there are drawbacks that a large number of steps are required, the production cost is high, problems regarding the waste liquid disposed in each step occur, and each step is not optimized and thus, changes in electric properties and optical properties before and after each step are accumulated.
The method disclosed in Patent Document 5 also has drawbacks that complete insulation of silver nanowires by the plasma treatment or the corona treatment is very difficult, and the base material is deteriorated by the plasma treatment or the corona treatment, resulting in decreasing the transparency and the durability.
Although no example regarding the silver nanowire is disclosed, Patent Document 6 discloses a following method as a method for printing a conductive ink on a substrate to form a patterned conductive layer.    (1) Forming a pattern on a substrate by printing, using a liquid repellent transparent insulation ink which contains a resin and a silicone-based or fluorine-based surfactant, and    (2) Coating a conductive ink which is repellent against a dry film of the liquid repellent transparent insulation ink, on the entirety of the dry film, and as the conductive ink is repelled by the dry film of the liquid repellent transparent insulation ink, a conductive ink layer is formed on a portion of the substrate where the dry film of the liquid repellent transparent insulation ink is not formed.
According to this method, both a repellent transparent ink and a hydrophilic conductive ink should be designed, and thus, the blending system becomes complicated, and the pattern accuracy largely depends on not only the printing accuracy, but also the repellencies between the repellent-hydrophilic resins. Therefore, improving the pattern accuracy is difficult.
Accordingly, it has been desired to directly forming a pattern by applying silver nanowires by a printing method such as inkjet printing, screen printing, gravure printing, and flexographic printing. However, in order to perform printing, a binder resin is necessary, and in order to maintain the transparent property, the amount of silver nanowires used should be reduced, which leads to drawbacks that the binder resin covers the surfaces of the silver nanowires and the conductivity is lost. When no binder resin is used, there are drawbacks that the pattern cannot be maintained at the printing, or even if the pattern can be maintained immediately after printing, the pattern is broken when the solution is dried.
Therefore, in Patent Document 7, an organic conductive polymer is used as a binder resin, and a pattern (transparent electrode) is formed according to the following steps.    (1) Step for forming a conductive layer pattern by gravure printing using a conductive ink which contains silver nanowires dispersed in a solution containing an organic conductive polymer as a binder    (2) Step for forming a transparent electrode by transferring the conductive layer pattern formed in the previous step on a second substrate.
According to this method, an organic conductive polymer is used for a binder, and thus, the resistance can be maintained at a low level to some extent. However, since a organic conductive polymer having a low shape-holding strength is used as a binder, a printing accuracy is not good, and the long-term reliability is low compared to a metal and an inorganic compound. Therefore, a practical quality as a transparent conductive film cannot be achieved.
Patent Document 8 discloses a method for forming a pattern using a water-soluble polymer such as a cellulose derivative having a Tg in the range of 0 to 250° C. as a binder resin, the method including the following steps.    (1) Step for printing a conductive ink on a substrate, the conductive ink containing silver nanowires dispersed in a solution which contains a water-soluble polymer such as a cellulose derivative having a Tg in the range of 0 to 250° C. as a binder resin.    (2) Step for eluting the water-soluble polymer by subjecting the conductive pattern to a heating and humidifying treatment under a pressurizing condition.
According to this method, no alkaline waste liquid is generated, but waste elution water may be generated. Further, uniformly pressurizing a conductive ink film is difficult, and conductivity is poorly-reproducible.